Labor activists vowed not to abandon their fight to organize college sports teams after the National Labor Relations Board blocked a historic bid by Northwestern University football players to become the first in the nation to unionize.

“It’s a setback, but I don’t see it as a body blow,” Tim Waters, of the United Steelworkers union, said about the ruling. The Steelworkers bankrolled the Wildcats’ union drive and advised them on strategy.

Labor agency rules offer no avenue for an appeal of Monday’s unanimous decision by the five-member board, which overturned a ruling last year by a regional NLRB director in Chicago that gave the Northwestern players the go-head to unionize.

Asked if one option would be for activists to file a new request to unionize on behalf of a team in another region, Waters said, “That is absolutely a possibility. … In no way, shape or form is this at an end.”

Ramogi Huma, a former linebacker at UCLA who worked closely with former Northwestern quarterback Kain Colter – the face of the union-building effort – said he hasn’t given up on bringing unions to college athletics, either.

“The door’s not closed,” he said.

Others said that optimism wasn’t justified.

Monday’s ruling emphasized that the NLRB has jurisdiction only over private schools, like Northwestern. All the other colleges in its Big Ten conference are public, and the board said giving individual private schools the OK to unionize would enable them to collectively bargain for advantages over state schools — such as more lucrative scholarships — potentially throwing off the delicate competitive balance in college sports.

Ronald Meisburg, a former general counsel for the NLRB and a onetime board member, said Monday’s ruling would serve as precedent and likely compel regional NLRB offices nationwide to flatly reject future petitions, no matter which union activists submit them.

“This puts the nail in the coffin of organizing college players,” he said.

An appeal to U.S. courts doesn’t appear to be an option for the losing side. Litigation in court is typically triggered by a company’s refusal to collectively bargain with a union approved by workers. That trigger won’t exist now that the NLRB has ruled out unionization by the Northwestern players.

The labor dispute goes to the heart of American college sports, where universities and conferences reap billions of dollars by relying on amateurs who are not paid. In other countries, college sports are small-time club affairs, while elite youth athletes often turn pro as teens.

Northwestern, in the Chicago suburb of Evanston, became the focal point of the labor fight in January 2014, when Colter announced plans to form the first U.S. labor union for college athletes.

Three months later, regional NLRB Director Peter Sung Ohr issued his decision, determining that Northwestern football players qualify as employees and thus should be able to unionize. A month later, players cast secret ballots on whether to form a union. But those ballots were sealed during the appeal and will now be destroyed without ever being counted.

Under U.S. law, an employee is regarded as someone who receives compensation for a service and is under the direct control of managers. In Northwestern’s case, Ohr concluded coaches are the equivalent of business managers and scholarships are a form of pay.

The NLRB didn’t address that portion of Ohr’s ruling Monday, drawing criticism from Waters, who said the board had sidestepped the most sensitive question.

“It’s like they had a hot potato tossed into their laps, and they took a year and a half of deliberations and said, `We’re going to toss it back,” he said.